This invention relates to a mask-forming device for forming a shadow mask of a color cathode ray tube, and more particularly to such a device using warm press forming.
In general, a color cathode ray tube has a built-in shadow mask with a color selection function within an envelope having a panel. Three electron beams for red, green and blue generated by the electron gun pass through minute apertures arranged precisely on the mask. As a result, the beams correctly strike the red, green and blue light-emitting phosphor deposited inside the panel, causing it to phosphoresce and produce a color image.
It is important in the mask that the minute apertures be precisely etched, and that the mask be formed to a precise curvature along the inside surface of the panel without any deformation. However, only one third or less of the total electron beams actually pass through the apertures of the mask, the rest striking and heating the mask.
In the prior art, aluminium-killed decarbonized steel generally has been used as the material for the mask. The coefficient of heat expansion of the aluminium-killed decarbonized steel is relatively high, and thus, it expands with heat and deforms, causing the beams passing through the apertures to be misaligned on the phosphor. In order to resolve this problem it has been suggested, as shown in Japanese Patent Publication No. 42-25446, that the mask be made of a material with a low thermal coefficient of expansion with heat, such as invar.
However when using invar alloy, which consists mainly of nickel-steel alloys, as the mask material, the yield point, which has a great effect on the mask-forming process, is 29-30 kg/mm.sup.2, as shown in Japanese Patent Laid-Open No. 59-200721. This is high compared with the yield point for aluminium-killed decarbonized steel, which is approximately 20 kg/mm.sup.2. Thus when forming a mask using a thin plate member of invar, with its high proportion of nickel-steel alloys, it is extremely difficult in a rectangular-shaped shadow mask to correctly form by press-forming alone, both a curved main section with a large number of apertures, and a rectangular skirt section. This is due to the springback tendency of invar plate after press forming.
However a distortion-free mask of high precision, which is made of a thin plate of invar alloy, can be obtained by warm press forming after aperture-etching and annealing, to lower the yield point. Warm press forming is a forming process whereby of a thin metal plate member is heated to a temperature lower than that of the recrystallization temperature of the plate member during plastic working.
However the above warm press forming is carried out with a press, and heating causes problems due to heat expansion of the forming device itself.
In carrying out warm press forming, electric heaters are built into at least the punch, the clamp, the knockout and the die of a press mold in order to heat the thin plate member as it is being pressed. In such a case, there may be a temperature difference between the die and its associated spacers, which confine the downward movement of the die. This temperature difference creates a pitch difference between guide pins fixed to the spacers, and guide bushes fixed to the die. This pitch difference is due to the difference in heat expansion between the die and the spacer. This pitch difference will, in turn lead to seizure, thus making it difficult for the press mold to operate as required.
Heat is also transferred to sliders of the press machine, which are in contact with the press mold, and heat expansion occurs in these sliders, as well. Thus, there is a change in the clearance between the slider and a fixed guide which supports movement of the slider, making it difficult to maintain the required accuracy. This is particularly true when a press mold that has been heating up for some time is attached to a press machine, since the heat expansion in the slider is even greater, causing it to seize against the fixed guide.
Furthermore, the heat from the heated parts of the device is conducted widely to other unheated parts, resulting in a very low heat efficiency, and more energy than necessary for heating must be supplied.
As explained above, in the prior art, heat is conducted to all the other parts of the apparatus when the press mold is heated, and seizing and the like occur due to differences in heat expansion of the various parts. Thus, it is difficult to carry out the process smoothly, and excessive energy must be supplied, which makes the process uneconomic.